A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 Hz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) for terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle one of these 63-sample PN sequences is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The other two 63-sample PN sequences and the 511-sample PN sequence are transmitted in accordance with the same logic convention in all data fields.
The data within data lines are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. The interleaved trellis codes have been subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission, which transmissions are made without symbol precoding separate from the trellis coding procedure. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without trellis coding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized vale of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
VSB signals using 8-level symbol coding will be initially used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than using VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.
It is assumed that the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for the QAM DTV signals. The VSB DTV signals modulate the amplitude of only one phase of the carrier at symbol rate of 10.76*10.sup.6 symbols per second to provide a real signal unaccompanied by an imaginary signal, which real signal fits within a 6 MHz band because of its VSB nature with carrier near edge of band. Accordingly, the QAM DTV signals, which modulate two orthogonal phases of the carrier to provide a complex signal comprising a real signal and an imaginary signal as components thereof, are designed to have a symbol rate of 5.38*10.sup.6 symbols per second, which complex signal fits within a 6 MHz band because of its QAM nature with carrier at middle of band.
Processing after symbol decoding is similar in receivers for the VSB DTV signals and in receivers for the QAM DTV signals, assuming the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for the QAM DTV signals. The data recovered by symbol decoding are supplied as input signal to a data de-interleaver, and the de-interleaved data are supplied to a Reed-Solomon decoder. Error-corrected data are supplied to a data de-randomizer which regenerates packets of data for a packet decoder. Selected packets are used to reproduce the audio portions of the DTV program, and other selected packets are used to reproduce the video portions of the DTV program.
The zero-intermediate-frequency (ZIF) receivers, which perform amplification and channel selection at baseband, that are used for receiving QAM DTV signals are not well suited for receiving VSB DTV signals. This is because of problems with securing adequate adjacent-channel rejection in a ZIF receiver when the carrier is not at the center frequency of the channel. The tuners can be quite similar in receivers for the VSB DTV signals and in receivers for the QAM DTV signals if the receivers are of superheterodyne types, however. The differences in the receivers reside in the synchrodyning procedures used to translate the final IF signal to baseband and in the symbol decoding procedures. A receiver that is capable of receiving either VSB or QAM DTV signals is more economical in design if it does not duplicate the similar tuner circuitry prior to synchrodyning to baseband and the similar receiver elements used after the symbol decoding circuitry. The challenge is in optimally constructing the circuitry for synchrodyning to baseband and for symbol decoding to accommodate both DTV transmission standards and in arranging for the automatic selection of the appropriate mode of reception for the DTV transmission currently being received.
DTV signal radio receivers are known of a type that uses double-conversion in the tuner followed by synchronous detection, having been used during field testing of the HDTV system used during development the ATSC standard. A frequency synthesizer generates first local oscillations that are heterodyned with the received VSB DTV signals to generate first intermediate frequencies (e. g., with 920 MHz center frequency and 922.69 MHz carrier). A passive LC bandpass filter selects these first intermediate frequencies from their image frequencies for amplification by a first intermediate-frequency amplifier, and the amplified first intermediate frequencies are filtered by a ceramic resonator filter that rejects adjacent channel signals. The first intermediate frequencies are heterodyned with second local oscillations to generate second intermediate frequencies (e. g., with 46.69 MHz carrier); and a filter, which can be of surface-acoustic-wave (SAW) type, selects these second intermediate frequencies from their images and from remnant adjacent channel responses for amplification by a second intermediate-frequency amplifier. The response of the second intermediate-frequency amplifier is supplied to a third mixer to be synchrodyned to baseband with third local oscillations of fixed frequency. The third local oscillations of fixed frequency can be supplied in 0.degree. phasing and in 90.degree. phasing, thereby implementing separate in-phase and quadrature-phase synchronous detection procedures during synchrodyning. Synchrodyning is the procedure of multiplicatively mixing a modulated signal with a wave having a fundamental frequency the same as the carrier of the modulated signal, being locked in frequency and phase thereto, and lowpass filtering the result of the multiplicative mixing to recover the modulating signal at baseband, baseband extending from zero frequency to the highest frequency in the modulating signal.
Separately digitizing in-phase and quadrature-phase synchronous detection results generated in the analog regime presents problems with regard to the synchronous detection results satisfactorily tracking each other after digitizing; quantization noise introduces pronounced phase errors in the complex signal considered as a phasor. These problems can be avoided in DTV signal radio receivers of types performing the in-phase and quadrature-phase synchronous detection procedures in the digital regime. By way of example, the response of the second intermediate-frequency amplifier is digitized at twice the Nyquist rate of the symbol coding. The successive samples are considered to be consecutively numbered in order of their occurrence; and odd samples and even samples are separated from each other to generate respective ones of the in-phase (or real) and quadrature-phase (or imaginary) synchronous detection results. Quadrature-phase (or imaginary) synchronous detection takes place after Hilbert transformation of one set of samples using appropriate finite-impulse-response (FIR) digital filtering, and in-phase (or real) synchronous detection of the other set of samples is done after delaying them for a time equal to the latency time of the Hilbert-transformation filter. The methods of locking the frequency and phase of synchronous detection and the methods of locking the frequency and phase of symbol decoding differ in the VSB and QAM DTV receivers.
These types of known DTV signal radio receiver present some problem in the design of the tuner portion of the receiver because the respective carrier frequencies of VSB DTV signals and of QAM DTV signals are not the same as each other. The carrier frequency of a QAM DTV signal is at the middle of a 6-MHz-wide TV channel, but the carrier frequency of a VSB DTV signal nominally is about 310 kHz above the lower limit frequency of the TV channel. Accordingly, the third local oscillations of fixed frequency, which are used for synchrodyning to baseband, must be of different frequency when synchrodyning VSB DTV signals to baseband than when synchrodyning QAM DTV signals to baseband. The 2.69 MHz difference between the two carrier frequencies is larger than that which is readily accommodated by applying automatic frequency and phase control to the third local oscillator. A third oscillator that can switchably select between two frequency-stabilizing crystals is a practical necessity. In such an arrangement, of course, alterations in the tuner circuitry are involved with arranging for the automatic selection of the appropriate mode of reception for the DTV transmission currently being received. The radio-frequency switching that must be done reduces the reliability of the tuner. The RF switching and the additional frequency-stabilizing crystal for the third oscillator increase the cost of the tuner appreciably.
Radio receivers for receiving digital television signals, in which receiver the final intermediate-frequency signal is somewhere in the 1-8 MHz frequency range rather than at baseband, are described by C. B. Patel et alii in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER, and included herein by reference. The use of infinite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,548,617 issued Aug. 20 1996, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The use of finite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,731,848 issued Mar. 24, 1998, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The design of receivers for both QAM and VSB signals in which QAM/VSB receivers both types of signal are processed through the same intermediate-frequency amplifiers receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. U.S. Pat. No. 5,606,579 issued Feb. 10, 1997 to C. B. Patel et alii and entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER is incorporated herein by reference. U.S. Pat. No. 5,659,372 issued Aug. 19, 1997 by C. B. Patel et alii and entitled DIGITAL TV DETECTOR RESPONDING TO FINAL-IF SIGNAL WITH VESTIGIAL SIDEBAND BELOW FULL SIDEBAND IN FREQUENCY is incorporated herein by reference. Allowed U.S. patent application Ser. No. 08/266,753 filed Jun. 28, 1994 by C. B. Patel et alii and entitled RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS is incorporated herein by reference. U.S. Pat. No. 5,715,012 issued Feb. 3, 1998 to C. B. Patel et alii and entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS is incorporated herein by reference. These patents and patent applications are all assigned to Samsung Electronics Co., Ltd., pursuant to employee invention agreements already in force at the time the inventions disclosed in these patents and patent applications were made.
In the QAM/VSB radio receivers described in U.S. Pat. Nos. 5,506,636 and 5,715,012 the final intermediate-frequency signal is digitized, and synchrodyne procedures to obtain baseband samples are carried out in the digital regime. A tuner within the receiver includes elements for selecting one of channels at different locations in a frequency band used for transmitting DTV signals, a succession of mixers for performing a plural conversion of signal received in the selected channel to a final intermediate-frequency (IF) signal, a respective frequency-selective amplifier between each earlier one of the mixers in that succession and each next one of said mixers in that succession, and a respective local oscillator for supplying oscillations to each of the mixers. Each of these local oscillators supplies respective oscillations of substantially the same frequency irrespective of whether the selected DTV signal is a QAM signal or is a VSB signal. The final IF signal is digitized, and thereafter there are differences in signal processing depending on whether the selected DTV signal is a QAM signal or is a VSB signal. These differences are accommodated in digital circuitry including QAM synchrodyning circuitry and VSB synchrodyning circuitry. The QAM synchrodyning circuitry generates real and imaginary sample streams of interleaved QAM symbol code, by synchrodyning the digitized final IF signal to baseband providing it is a QAM signal and otherwise processing the digitized final IF signal as if it were a QAM signal to be synchrodyned to baseband. The VSB synchrodyning circuitry generates a real sample stream of interleaved VSB symbol code, by synchrodyning the digitized final IF signal to baseband providing it is a VSB signal and otherwise processing the digitized final IF signal as if it were a VSB signal to be synchrodyned to baseband. A detector determines by sensing the presence of a pilot carrier accompanying a DTV signal of VSB type whether or not the final IF signal is a VSB signal to generate a control signal, which is in a first condition when the final IF signal apparently is not a VSB signal and is in a second condition when the final IF signal apparently is a VSB signal. Responsive to the control signal being in its first condition, the radio receiver is automatically switched to operate in a QAM signal reception mode; and responsive to the control signal being in its second condition, the radio receiver is automatically switched to operate in a VSB signal reception mode.
U.S. Pat. Nos. 5,506,636, 5,715,012 and U.S. patent application Ser. No. 08/266,753 were written presuming that the carrier frequency of a VSB DTV signal would be 625 kHz above lowest channel frequency, as earlier proposed by a subcommittee of the Advanced Television Systems Committee. This specification presumes that the carrier frequency of a VSB DTV signal is about 310 kHz above lowest channel frequency, as specified in Annex A of the Digital Television Standard published Sep. 16, 1995.
The carrier of the final IF signal is preferably one prescribed subharmonic of a multiple of the symbol frequencies of both the QAM and VSB signals if the selected DTV signal is a QAM signal and is another prescribed subharmonic of that multiple if the selected DTV signal is a VSB signal. When the carrier frequency of a VSB DTV signal is nominally 310 kHz above lowest channel frequency, these prescribed subharmonics should differ in frequency by substantially 2.69 MHz. Digitizing the final IF signal at this multiple of the symbol frequencies of both the QAM and VSB signals facilitates the generation of the digital carriers used to synchrodyne the QAM and VSB final IF signals to baseband. This multiple of the symbol frequencies of both the QAM and VSB signals should be low enough that digitization is practical, but is preferably above Nyquist rate.
In one type of these QAM/VSB radio receivers the prescribed subharmonic of a multiple of the symbol frequency of the QAM signal is substantially 2.69 MHz higher in frequency than the prescribed subharmonic of a multiple of the symbol frequency of said VSB signal. In a preferred such receiver the frequency of the QAM carrier in the final IF signal is 5.38 MHz, the first subharmonic of 10.76 MHz, and the frequency of the VSB signal carrier in the final IF signal is 2.69 MHz, the third subharmonic of 10.76 MHz.
In another type of these QAM/VSB radio receivers the prescribed subharmonic of a multiple of the symbol frequency of the QAM signal is substantially 2.69 MHz lower in frequency than the prescribed subharmonic of a multiple of the symbol frequency of the VSB signal. The VSB signal having its full sideband below carrier frequency in the final IF signal is sampled with better resolution in such embodiments of the invention. In a preferred such embodiment the frequency of the QAM carrier in the final IF signal is 5.38 MHz, the first subharmonic of 10.76 MHz, and the frequency of the VSB signal carrier in the final IF signal is 8.07 MHz, the third subharmonic of the third harmonic of 10.76 MHz.
When synchrodyning is done in the digital regime, the generation of the digital carriers from read-only memory (ROM) is facilitated by digitizing the final IF signal of both the QAM and VSB signals at a sampling rate that is a multiple of each of their symbol rates. Phase-locking the frequency of the carrier used for synchrodyning to the carrier of the QAM or VSB signals to baseband is thereby facilitated.
Digitizing the QAM and VSB DTV signals at multiples of their symbol rates facilitates symbol synchronization whether synchrodyning is done in the digital regime as described by Patel et alii or is done in the analog regime. In order to perform symbol synchronization satisfactorily, digital samples must be provided at a sample rate at least twice symbol rate. Supplying digital samples at a rate higher than symbol rate will increase the number of taps in the digital filters used for channel equalization of the baseband DTV signal, since the number of sample times in a ghost of any particular duration will increase in direct ratio to how many times the symbol rate the sampling rate is. Digitizing a QAM or VSB DTV signal at a multiple M times N of its symbol rate, M being a positive number at least one and N being a positive integer at least 2 allows the N:1 decimation of the digital DTV baseband signal before performing channel equalization thereof, so long as the Nyquist criterion for transmitting symbols is satisfied in the decimated digital signal.